The employment of mass spectrometry for identification of chemical structures, molecular weights, determination of mixtures, and quantitative elemental analysis, based on the application of the mass spectrometer, is a known analytical technique. Mass spectrometry may be used to accurately determine the molecular weights and structural information of organic molecules based on the augmentation pattern of molecular fragments and the ions formed when the molecule undergoes ionization. The weights of molecules may be measured by ionizing the molecules and measuring their trajectories in response to electric and magnetic fields in a vacuum.
Organic molecules having a molecular weight greater than about a few hundred to few thousand are of great medical and commercial interest as they include, for example, peptides, proteins, DNA, oligosaccharides, commercially important polymers, organometallic compounds and pharmaceuticals. Large organic molecules, of molecular weight over 10,000 Daltons, may be analyzed in a quadrupole mass spectrometer using "electrospray" ionization to introduce the ions into the spectrometer.
Electrospray mass spectrometry (ESI/MS) has more recently been recognized as a significant tool used in the study of proteins and protein complexes. Electrospray ionization as a method of sample introduction for mass spectrometric analysis is also known. Generally, electrospray ionization is a method whereby ions are formed at atmospheric pressure and then introduced into a mass spectrometer using a special interface. In electrospray ionization, a sample solution containing molecules of interest and a solvent is pumped through a hypodermic needle and into an electrospray chamber. An electrical potential of several kilovolts may be applied to the needle for generating a fine spray of charged droplets. The droplets may be sprayed at atmospheric pressure into a chamber containing a heated gas to vaporize the solvent. Alternatively, the needle may extend into an evacuated chamber, and the sprayed droplets then heated in the evacuated chamber. The fine spray of highly charged droplets releases molecular ions as the droplets vaporize at atmospheric pressure. In either case, ions are focused into a beam, which is accelerated by an electric field gradient, and then analyzed in a mass spectrometer.
Because electrospray ionization occurs directly from solution at atmospheric pressure, the ions formed in this process tend to be strongly solvated. To carry out meaningful mass measurements, it is necessary that any solvent molecules attached to the ions be efficiently removed, that is, the molecules of interest must be "desolvated". In the prior art, desolvation is achieved in one way by interacting the droplets and solvated ions with a strong countercurrent flow (6-9 l/m) of a heated gas before the ions enter into the vacuum of the mass analyzer.
The use of such a strong countercurrent gas flow is expensive and difficult to operate because the gas flow rate and the temperature need to be controlled precisely and be optimized for each analyte and solvent system. If proper gas flow and temperature conditions are not attained, it can result in either an incomplete desolvation of the ions or a decrease in sensitivity as ions may be swept away by the gas at high flow rate. To enhance the desolvation process, some have used collisional activation by applying an electrostatic field in a region of reduced pressure between the sampling orifice of the mass analyzer and the skimmer.
Although high speed pumping is commonly incorporated to allow for the direct sampling of electrosprayed ions into the mass analyzer, the detailed method of ion transport from atmospheric pressure to vacuum is different in each case. Thus ion transport has been achieved through a 0.2 mm bore 60 mm long glass capillary tube and skimmer and a 1.0 mm diameter sampling orifice and skimmer.
Chowdhury et al. disclose in U.S. Pat. No. 4,977,320 a modified mass analyzer connected to an electrospray ion source to form a mass spectrometer. The ion source employed by Chowdhury et al. includes a syringe needle having a high voltage (4-6 KV) imposed upon it and having an exit orifice spaced in ambient atmosphere of the laboratory at a distance (0.5-4.0 cm) from the entrance orifice of a long metal capillary tube. The capillary tube is heated (80.degree.-90.degree. C.) by an electrical resistance coil and held at a lower voltage (under 400 V). The exit orifice of the capillary tube is separated from a skimmer and is disposed within a vacuum chamber having a pressure of about 1-10 torr. A hole (0.5 mm dia.) in the skimmer leads to a second vacuum chamber (4.times.10.sup.-4 torr), to a series of lenses, each with a hole therethrough, and finally to a baffle having a hole (2.4 mm dia.) therethrough leading to the vacuum chamber (2.times.10.sup.-5 Torr) of the mass analyzer (quadrupole analyzer).
In Chowdhury et al., the molecules of interest, a protein, for example, are dissolved in a solvent or mixture of solvents and the solution is then pumped through the syringe needle. The solution is then electrosprayed therefrom in micron size droplets into the atmosphere so it may be viewed and adjusted by the user. The electric field in the gap between the electrospray syringe needle and the entrance orifice of the capillary tube causes the formation of charged droplets that enter the capillary tube. The strong flow of gas in the capillary tube as a result of the pressure differential between the ends of the tube causes the charged droplets to progress down the center of the tube. Heating of the capillary tube causes evaporation of the droplets and desolvation of the resulting molecule ions of interest. (Chowdhury et al. state that the capillary tube may be heated by an electrical resistance wire wound about the tube or the tube may be a resistive heating element.) The ions then exit into a vacuum chamber where solvent is further removed by collisional activation and then the charged ions pass through the hole in the skimmer, through the holes in the lenses and baffle and ultimately into the spectrometer.
In U.S. Pat. No. 5,015,845, Allen et al. disclose an electrospray method for mass spectrometry wherein a high voltage is applied to a capillary tube for receiving spray droplets containing sample solute of interest and solvent at substantially atmospheric pressure or above. The electrosprayed droplets are passed into an ion generating chamber which is maintained at a pressure in the range of about 0.1 to 10 torr. The walls of the ion generating chamber are controllably heated to a temperature that desolvates the droplets and produces ionized molecules of interest for analysis by the mass spectrometer.
Chowdhury et al. state that it is an object of their invention to provide an ion source that will fit on commercial mass analyzers with only minor modifications; however, a need exists for an effective electrospray ionization source compatible with commercial mass analyzers having standard 0.5 inch (13 mm) vacuum locks thereby requiring no modifications.